Book excerpt: Flex 4 is an open-source tool that allows developers to easily add life to web applications with dynamic user features, colorful transitions, and eye-catching animations. Flex also provides powerful data handling for industrial-strength applications. We think it should be just as much fun to learn Flex as it is to use it. And we know that fun learning gets better results. Hello! Flex 4 demonstrates how to get started without getting bogged down in technical detail or academic edge cases. In this book, User Friendly cartoon characters offer commentary and snide side comments, as the book moves quickly from Hello World into practical techniques. Each one is illustrated with a hands-on example. Along the way, readers will build a unique Flex application that mashes Yahoo Maps with Twitter to keep track of friends. Purchase of the print book comes with an offer of a free PDF, ePub, and Kindle eBook from Manning. Also available is all code from the book.